The present invention relates to a color copier, color printer, color facsimile apparatus or similar color image forming apparatus.
A color printer, for example, transfers color toners to a printing medium or sheet by using an electrophotographic process and thereby forms a color image. Specifically, the printer charges the surface of a photoconductive element with a charger, electrostatically forms a latent image on the charged surface of the photoconductive element with an exposing unit, develops the latent image with a color toner, transfers the color toner to a sheet with an image transfer unit, and then fixes the toner on the sheet with a fixing unit.
The problem with the conventional color printer is that the density and tonality of a particular color toner a r e degraded due to, e.g., the aging of the photoconductive element or image carrier and the varying temperature an d humidity around the printer. This, coupled with the fact that the color balance of the resulting color image is also degraded, lowers the print quality to a critical degree. In order to eliminate this problem, it has been customary with, e.g., a digital color copier, to form multitone test patterns by toners of different colors, optically read the individual test pattern by a photosensor, determine the toner density of the individual test pattern, and then adjust a voltage to be applied to a charger and/or a voltage to be applied to a developing unit so as to correct the toner density.
Assume that the multitone test pattern whose toner density ranges from 0% to 100% in area ratio is printed on a sheet by a color toner and implemented as a dot image, and then read by a photosensor. Then, the output of the photosensor is lowest at the toner density of about 50% while increasing in ranges higher than and lower than about 50%. This stems from the fact that the color toner reflects incident light in a particular spectrum. Specifically, in the portion of the test pattern where the density is low, the reflection from the toner decreases because the area of the background decreases with an increase in toner density. In the medium density portion, the decrease in reflection attributable to the decrease in background area and the increase in reflection attributable to the increase in toner area are balanced. Further, in the high density portion, the reflection increases due to an increase in toner area. Hence, even when such a multitone test pattern is formed by the color toner and then read by the photosensor, the actual toner density of the pattern cannot be determined with accuracy.
In light of the above, there has been proposed an image forming apparatus capable of determining the toner density of a color toner by use of a photosensor responsive to a test pattern, and a potential sensor responsive to the surface potential of a photoconductive element. This kind of scheme, however, needs two different sensors, and moreover results in a complicated structure due to the decision based on the outputs of the two sensors. The complicated structure obstructs the miniaturization of the apparatus and lowers productivity.